The present invention concerns an open injection-molding nozzle with a needle exhibiting multiple tips on its mold-facing side for injection molding of a shaped piece with several sprue marks, each tip, when in the operating position, forming an annular slot in cooperation with an entrance opening in a cavity of the mold.
Such an open injection-molding nozzle is known and has proven itself. For example, such a nozzle can be used in molding parts in which a central sprue mark is not possible since the finished part will possibly later have an opening at this point. With a multi-tip needle, several sprue marks can be arranged around such an opening.
Problematic in the case of such open injection-molding nozzles is especially injection during the final thrust phase in which the mold or the cavity located in the mold is already substantially filled and only a little injection-molding material can be injected under high pressure since the annular slots around the individual needle tips cause setting of the injection-molding material (usually plastic) due to the decreasing flow rate. Thus, open injection-molding nozzles have usually been suitable up to now only for low-cost parts.
Indeed, one could make the annular slot at a given tip larger; however, a fractured sprue would result upon opening of the mold, which is not acceptable in the case of many injection-molded parts.
The problem is therefore to produce an injection-molding nozzle of the type mentioned above in which injection-molding material can still be introduced even in the final thrust phase while attaining a clean sprue break.
In solving this problem, the injection-molding nozzle of the invention is characterized by the fact that the needle located inside the open nozzle is shiftably secured in its longitudinal direction and can be retracted to enlarge the annular slots and openings located in the area of the needle tip.
Thus, for example, one can first inject with a relatively narrow annular slot at a high flow rate, which has the advantage that a large amount of frictional heat simultaneously develops in the annular slots, which favors and maintains the flow process. If the mold or the cavity in the mold is almost filled, however, and if the injection-molding material begins, on the one hand, to solidify and, on the other hand, to shrink, so that a final thrust phase is required in which, however, the frictional heat is no longer available due to the considerably reduced flow rate, the needle can be retracted in accordance with the present invention and the annular slot enlarged as a result so that injection-molding material can be appropriately introduced even in the final thrust phase under the less favorable conditions involved. Just prior to ending such an injection-molding process, the needle can again be shifted in the longitudinal direction so that the annular slot is of an appropriately small size upon opening of the mold and a correspondingly clean break occurs.
Through the present invention, however, it is also possible to retract the needle already shortly after the initial phase of the injection process until even the final thrust phase has been completed and only then to shift the needle back again in order to create conditions favorable for mold opening.
It is especially appropriate if a drive for reciprocating movement, for example, a piston drive, a positioning motor, a piezoactor, or the like, acts on an extension of the needle located outside the nozzle. In this way, the adjusting movements of the needle provided according to the present invention in the open injection-molding nozzle can be nicely carried out.
As a result, in a further development of the present invention, the reciprocating movement of the needle can be controlled and the tips of the needle can be adjustable or adjusted at the beginning of the injection molding process far or deep into the openings, reducing the size of the annular slot and resulting in the already mentioned high flow rate.
The adjustable needle, as already briefly mentioned, can be retracted after an initial phase of the injection-molding process through the end of the final thrust phase and then shoved back deeper into the sprue openings prior to opening the mold in order to attain the best possible matching of needle position to the injection-molding process.
Especially upon combination of one or more of the prescribed features and measures, the advantages of a relatively economical open injection-molding nozzle can be combined with the requirement for permitting a good final thrust phase and still obtaining a clean break upon opening the mold.